Phase measuring devices are used to determine the phase difference between two signals, normally a reference signal and a signal being measured. Traditionally, these devices have been large, bulky analog circuits which have a high level of complexity and incorporate a large number of components.
Since the phase of the output of all analog components is a function of the amplitude of the signal applied to the component, any change in the amplitude of the signal applied to the component changes the phase of the output signal itself. This is quite troublesome in a system designed to determine distance via accurate phase measurement.
Due to the number of components required to construct one of these analog systems, they tend to be quite expensive and consume i high level of power. Further, due to its internal analog electronics, the output signal of these devices is not easily interfaced with today's digital equipment. Additionally, due to the inaccuracy of the analog components used to construct these devices, the smallest unit of phase shift these devices are usually capable of measuring is 1.degree..
In an attempt to improve these analog devices, a digital output stage was attached to the analog phase measuring device. However, this only increased the number of components required to produce such a device and, additionally, increased the power consumption and cost of such devices. Unfortunately, this modification did not greatly enhance the accuracy of the device, as the smallest unit of phase shift measurable was only reduced to 0.1.degree..
In the advent of the personal computer, purely digital phase measurement systems were created. These devices typically incorporate a two channel analog to digital converter card which is installed in a personal computer. The two signals (the reference signal and the measured signal) are fed into the analog to digital converter card and the microprocessor of the personal computer runs a program which determines the phase difference between the two signals. While this device is purely digital, its data processing rate is typically limited to only 100 kilohertz. Additionally, these devices are very large and bulky, consume a high level of power, and are not easily moved from site to site. As with the analog systems previously disclosed, this purely digital system is unable to unambiguously determine phase shift when the phase shift exceeds 359.999.degree..